my sister and i bith used to be like this too. i remember one time i fell asleep at 4pm after school and woke up at 1 the next afternoon. you could possibly be anemic? that's why she was like that. i was like this because of depression and just habit.
try staying away from someplace you will fall asleep, take a walk and get some energy, if you can break the habit of napping you will be better off.
I would try to boost your energy a litle bit every morning. Try eating a big breakfast before you go to school in the morning, to give you a little more energy, or take in more caffene everyday. Also, if its really bothering you that bad, when you come home from school, do what you have to do, and then go out until you have to be in that night. Being active is probably your best bet.
I use to stay awake all night and then sleep during the day, which wasnt good for my schedule, and it really wasnt healthy. but sleeping all day except for when your in school sounds more serious, if what i suggested doesnt work, or you've all ready tried those things, i would have your mom or dad call your doctor, there could be something wrong